In the construction of semiconductor integrated circuits, several types of topside layers are currently in use. This topside "passivation layer" is used as a dielectric barrier to protect the underlying circuitry of the integrated circuit from both moisture and contaminants, which can cause corrosion or electric shorts.
In the early development of memory-type integrated circuits, a need became apparent for reprogrammable cells to accomodate project and program development during which specifications and performance criteria often change. This need has been largely supplied by the ultraviolet (UV) radiation erasable programmable read only memory (EPROM) integrated circuit.
Basically, the erase feature is provided by shining ultraviolet light onto the semiconductor chip. This excites the electrons trapped on a floating gate region of the circuit structure and causes the electrons to move off the floating gate.
Obviously, in order to perform the erase function the topside passivation layer of the integrated circuit must be transparent to UV light to a degree sufficient to allow the energy levels of the trapped electrons to be raised to a state where they can diffuse off the gate.
Unfortunately, the only way to remove the trapped gate electrons from the EPROM memory cell is by high intensity UV irradiation through a relatively high-cost clear quartz window, or other UV transparent layer, on the top of the package upon which the chip is mounted.
Silicon nitride is considered to be one of the best compositions for use as a passivation layer on semiconductor integrated circuits as a dielectric. It is known to have a high resistance to moisture and hydrogen penetration which would ruin the circuit. Moreover, diffusivity of various impurities, such as sodium, is much lower in silicon nitride than in other insulators, such as silicon dioxide. Thus, integrated circuits made with a silicon nitride passivation layer are less susceptible to ionic contamination problems.
Silicon nitride is generally deposited by combining silane (SiH.sub.4) and ammonia (NH.sub.3) by the following chemical vapor deposition (CVD) reaction: EQU 3SiH.sub.4 +4NH.sub.3 .fwdarw.Si.sub.3 N.sub.4 +12H.sub.2
This pyrolytic reaction can take place at atmospheric pressure both with nitrogen (at approximately 650.degree. C.) or hydrogen (at approximately 1000.degree. C. ) as carrier gases. However, these temperatures are impractical for deposition of topside films.
Plasma deposition involves the use of gas reactants, in an evacuated reactor, that are converted into very reactive chemical species with the help of an RF glow discharge and result in vapor deposition of the desired material. Thus the plasma-deposition process is a variation of the CVD process in which a gas plasma replaces the heat-induced decomposition reactions. The most significant difference lies in the lower temperature of the plasma deposition process (approximately 200.degree. to 450.degree. C.), which does not cause a phase change of the underlying metallic alloy film and does not alter the junction depth of transistors.
However, it is commonly believed that silicon nitride deposited by plasma techniques is opaque to UV light at 2537 Angstroms (the wavelength of mercury UV light used to erase EPROM devices). See e.g., K. Alexander, et al., Moisture Resistive, U.V. Transmissive Passivation for Plastic Encapsulated EPROM Devices, IEEE International Reliability Physics
Symposium, Las Vegas (1984); J. K. Chu, et al., Plasma CVD Oxynitride as a Dielectric and Passivation Film, Electrochem. Soc. Ext. Abstr., Vol. 83-2, Abstract No. 321 (1983); M. J. Rand and D. R. Wonsidler, Optical Absorption as a Control Test for Plasma Silicon Nitride Deposition, J. Electrochem. Soc. Vol. 125, No. 1, 99 (1978).
Therefore, silicon nitride has been considered unsuitable for use as a passivation layer on UV EPROM devices.
Specifically, with respect to plasma deposited silicon nitride films, it has been reported recently that the refractive index of silicon nitride films is correlated only to the silicon-to-nitrogen ratio created in the reaction chamber, but that there is no good correlation between the silane-to-ammonia ratio and the refractive index. T. E. Nagy, et al., Silicon Nitride Thin Insulating Films, Electrochem. Soc. Proceedings, Volume 83-8, (1983).
Based upon these factors, silicon oxynitride films with refractive indices below 1.80 are widely considered to be the films of choice as passivation layers for EPROM packaging. Processes have been developed to yield oxynitride films with refractive indicies of approximately 1.70.+-.0.1. However, erase times of films created in some commercially available plasma deposition systems were found to be much shorter than others. This result, in conjunction with data on integrity of the films, suggested that deposition conditions strongly affect film characteristics. A wide range of deposition parameters, including flows of each participant gas in the reaction, temperature, pressure, RF power and frequency, as well as geometry of the different systems, were thus subject to the investigation leading to the present invention.
Hence, it is an object of the present invention to provide a method of forming a silicon nitride film which is transparent to ultraviolet light.
It is another object of the present invention to provide a plasma deposition method of forming a silicon nitride film for use in the production of semiconductor integrated circuits.
A further object of the present invention is to provide an improved silicon nitride passivation layer for EPROM integrated circuits.
Yet a further object of the present invention is to provide a method of forming a UV light transparent silicon nitride film on an EPROM semiconductor integrated circuit.